1. Field of the Invention
The present invention relates to an aperture stop mechanism for adjusting the size of an aperture through which light is passed. The present invention also relates to an illumination optical system, a projection optical system, and a projector provided with such an aperture stop mechanism.
2. Description of Related Art
In many optical apparatuses including cameras, it has conventionally been quite common to adjust the diameter of a light beam by intercepting the light traveling in a peripheral portion of the light beam by the use of an aperture stop mechanism having an aperture. For example, an aperture stop mechanism designed for use in a camera which permits the size of an aperture to be varied is disclosed in Japanese Patent Application Laid-Open No. 2003-57715.
Today, in projectors that display an image and then project the displayed image onto a screen, it is often permitted to switch between high-brightness settings, with which the image is presented with high brightness, and high-contrast settings, with which the image is presented with high contrast, according to the environment in which the projectors are used. For this purpose, an aperture stop mechanism that permits the size, shape, and position of an aperture to be varied is used.
A projector typically adopts, in the display section thereof for displaying an image, either a display device, such as a cathode-ray tube (CRT), that itself emits light that represents the image or a display device, such as a liquid crystal display (LCD) or digital micromirror device (DMD, a trademark of Texas Instruments Incorporated), that produces image light by modulating illumination light that is fed thereto. In a projector of the former type, an aperture stop mechanism is disposed in a projection optical system that projects the image light that emanates from the display section; in a projector of the latter type, an aperture stop mechanism is disposed in an illumination optical system that feeds illumination light to the display section, or in a projection optical system that projects the image light that emanates from the display section, or one in each of the illumination optical system and the projection optical system.
A conventional common aperture stop mechanism that permits the size and shape of an aperture to be varied is shown in FIGS. 8A to 8C. FIG. 8A is a front view with the aperture 90 fully stopped, and FIGS. 8B and 8C are a front view and a side view, respectively, with the aperture 90 fully open. This aperture stop mechanism includes an aperture base plate 91 and an aperture cover 93, with a blade chamber formed therebetween. In the blade chamber, a drive ring 92 is rotatably arranged. Between the aperture base plate 91 and the drive ring 92, an aperture blade 94 is arranged, and, between the aperture cover 93 and the drive ring 92, an aperture blade 95 is arranged.
The aperture base plate 91, aperture cover 93, and drive ring 92 each have a large circular opening at the center thereof. Along the rim of the opening of the aperture base plate 91, a wall that is elevated therefrom is formed for guiding the rotation of the drive ring 92. At the outer edge of the drive ring 92, a gear 92a is formed for receiving a driving force from a motor 96.
The aperture blade 94 has a pin 94a on the surface thereof facing the aperture base plate 91, and has a pin 94b on the surface thereof facing the drive ring 92. The pin 94a is inserted in a hole formed in the aperture base plate 91, and the pin 94b is inserted in a cam groove 92b formed in the drive ring 92. Thus, the aperture blade 94 can rotate about the pin 94a. The aperture blade 95 has a pin 95a on the surface thereof facing the aperture cover 93, and has a pin 95b on the surface thereof facing the drive ring 92. The pin 95a is inserted in a hole formed in the aperture cover 93, and the pin 95b is inserted in a cam groove 92c formed in the drive ring 92. Thus, the aperture blade 95 can rotate about the pin 95a. 
When the motor 96 is driven, a motor gear 97 rotates, and, since this meshes with the gear 92a, the drive ring 92 rotates together. Thus, the pins 94b and 95b slide along the cam grooves 92b and 92c, causing the aperture blades 94 and 95 to rotate about the pins 94a and 95a. As a result, the size of the aperture, of which the rim is defined by the aperture blades 94 and 95, varies.
The aperture blades 94 and 95 can move completely out of the opening of the aperture base plate 91, and thus the rim of the fully open aperture is defined by the aperture base plate 91. Accordingly, the aperture blades 94 and 95 define the rim of the aperture except when it is fully open. The drive ring 92 only rotates, and remains in the same place all the time. Thus, the drive ring 92 takes no direct part in defining the rim of the aperture.
As described above, the conventional aperture stop mechanism includes an aperture base plate, an aperture cover, a drive ring, a plurality of aperture blades, a motor, and a motor gear; that is, it requires a large number of components and has a complicated construction. Fabricating these components and assembling them together requires much cost and time, and in addition the complicated construction is prone to failure. Moreover, since the aperture blades are moved by the action of the drive ring, if the cam grooves formed therein are shaped with low accuracy, the positions of the aperture blades do not exactly correspond to the amount of rotation of the motor. This makes it impossible to adjust the aperture to the desired size and shape.